In recent years, coronary artery disease (CAD) has become a major cause of death worldwide.1 The mechanisms through which the disease occurs, including angina pectoris, myocardial infarction, ischemic cardiomyopathy, and occult CAD, are not fully understood and are still being predominantly defined only theoretically. Endothelial cell damage, mononuclear macrophage and vascular smooth muscle cell conversion to foam cells, and vascular inflammation are all inextricably linked to the development of CAD). Highly sensitive and specific markers capable of predicting its occurrence and reducing the risk of cardiovascular mortality are needed. The intestinal flora, an important metabolic organ in the body, allegedly promotes coronary atherosclerosis.2,3 Trimethylamine oxide (TMAO) is a metabolite of the intestinal flora, and studies have comprehensively shown that plasma TMAO and its precursors can predict the risk of cardiovascular disease in humans,4–6 even outperforming lipoproteins and conventional lipid parameters in predicting unstable angina.7 However, whether TMAO can be a more reliable clinical predictive marker for coronary heart disease remains unknown.
INTESTINAL FLORA AND TMAO
The human intestine harbors more than 1000 species of bacteria, the total number of which is approximately identical to the total number of human cells. As the larger endocrine organ of the body, the intestine can produce essential vitamins for the body and participate in critical metabolic processes such as folic acid synthesis, vitamin B12 synthesis, and bile acid metabolism. The intestinal flora is pivotal to promoting the development of the immune system, maintaining normal immune function, and resisting pathogen invasion.8 For example, the presence of actiDnomycetes correlates negatively with cholesterol levels in humans, Bifidobacterium improves glucose tolerance, and fungal bacteria are associated with visceral fat content in humans.9 The presence and abundance of specific intestinal microorganisms can be used as potential predictive biomarkers for the diagnosis of acute coronary syndrome (ACS).10 Researchers have established connections between intestinal flora metabolites and inflammatory bowel disease,11 depression,12 etc. Others have demonstrated that some intestinal flora metabolites are harmful to humans; for example, endotoxin is a risk factor for coronary heart disease, heart failure, and atrial fibrillation;,13 indoxyl sulfate (IS) acts as a uremic toxin and cardiotoxin in the human body;14 and TMAO is linked to the processes of coronary heart disease, heart failure, diabetes, chronic kidney disease, and many other disorders.15–19
Red meat, fish, shellfish, seafood, eggs, dairy products, and beans are rich in choline, l-carnitine, and betaine,20 components that participate in the production of TMAO. Choline is cleaved by the choline trimethylamine–lyase system (CutC/CutD) of the intestinal flora to produce the precursor substance TMAO21; l-carnitine is converted to TMA by the CutA/CutB system produced by the intestinal flora, and also first to betaine and then to TMA by the yeaW/yeaX enzyme system derived from the intestinal flora.22 Betaine is catalyzed by l-carnitine dehydrogenase and then reduced to TMA by betaine reductase.21 After production, TMA enters the liver through the portal vein, where it is further oxidized to TMAO. Flavin monooxygenase (FMO) in the liver is extremely important for the conversion of TMA to TMAO; there are 3 types of FMO: FMO1, FMO2, and FMO3. FMO3 is more active and therefore more crucial to the conversion of TMA.23 Generated TMAO is distributed to the liver, brain, muscle, kidney, and intestine, and takes part in several metabolic pathways, favorably or unfavorably affecting organisms. TMAO content rises within 4–8 hours of ingesting choline-rich food and is excreted after 24 hours through glomerular filtration or renal tubular epithelial secretion; as a result, the circulating level of TMAO depends, to some extent, on renal clearance24(Fig. 1).
TMAO and Disease
TMAO is reportedly associated with the development and progression of several cardiovascular diseases, such as coronary heart disease, hypertension, heart failure, etc.25–29 In patients with cardiovascular disease, elevated plasma TMAO levels can increase the risk of adverse cardiovascular events (MACE).4,15,30–32 A meta-analysis found that each 10-μmol/L increase in TMAO levels was accompanied by a 7.6% increase in the risk of cardiovascular mortality.33 Interestingly, the predictive value of TMAO for CAD tends to be more prominent. One prospective study of 292 patients with ACS demonstrated the high sensitivity (85%) and specificity (80%) of high TMAO levels (>4 μmol/L) in predicting cardiovascular mortality, with the negative predictive value of TMAO, in particular, potentially as high as 99%.34 A large independent clinical cohort study demonstrated for the first time an independent relationship between plasma TMAO levels and calcification in culprit lesion segments using OCT in patients with ST-segment elevation myocardial infarction (STEMI). TMAO has been reported to be associated with plaque instability, which are key risk factors for malignant cardiovascular events.35 A cross-sectional study showed that TMAO is also strongly linked to an increased number of coronary infarctions36 and is a significant predictor of the 5-year all-cause mortality risk in patients with stable peripheral arterial disease.37
TMAO is excreted through the kidneys and is among the major uremic toxins that contribute to chronic kidney disease (CKD). It promotes renal fibrosis, with its circulating levels capable of predicting survival in patients with CKD.38 One large community-based cohort study of older adults found that TMAO levels were associated with a high risk of coronary heart disease and a significantly higher risk of coronary heart disease due to impaired renal function.39 Another investigation noted link between TMAO levels and all-cause mortality in dialysis patients.40
Demonstrably, TMAO levels correlate with diabetes severity.41An elevated TMAO concentration predicts a high risk of death in controlled type 2 diabetes.42 TMAO production is reduced in people with FMO3 mutations, with the accumulated nonconverted TMA spreading to other parts of the body before being eventually excreted through respiration and sweat, leading to fish taste syndrome.43 In a nutshell, circulating TMAO levels are inextricably linked to several diseases and could be a crucial causative factor.
Despite its adverse effects, TMAO has also been shown to exhibit some protective properties in humans. Experiments with some animal models have revealed that TMAO reduces blood pressure and plasma levels of NT-proBNP in rats and also inflates the left ventricular ejection fraction (LVEF).44TMAO has a protein-stabilizing effect and promotes heavy plasmin and actin complexes, increasing myocardial contractility in humans and mice.45 It is believed to be beneficial to the health of deep-sea animals because it protects proteins from hydrostatic stress.33
Plasma TMAO concentrations correlate negatively with soft plaque, total plaque load, and calcified plaque load.46 TMAO may also act as a molecular chaperone that limits the stress response of the endoplasmic reticulum in injured cells by inhibiting the activation of unfolded proteins. In addition, low amounts of TMAO possibly play a protective role in coronary atherosclerosis.15 One study on a Chinese population reported no significant association between TMAO concentrations and ACS.47 Another inquiry determined that using TMAO levels to differentiate symptomatic from asymptomatic cerebrovascular diseases was not practical.48 TMAO levels may also not affect the progression of early atherosclerosis.49The pros and cons of TMAO remain debatable, with most studies currently associating TMAO with exacerbating disease progression. TMAO's relationship with diseases must be investigated further.
TMAO/TMAO Precursors and Coronary Artery Disease
TMAO Promotes Vascular Endothelial Inflammation
The vascular endothelium is essential for maintaining the physiological function of the cardiovascular system. The endothelium acts as a barrier to plaque formation, with endothelial integrity a necessity for antithrombotic events. TMAO promotes the endothelium's inflammatory response through multiple mechanisms: it stimulates vascular inflammation and exacerbates vascular endothelial cell injury by activating mitogenic protein kinase (MAPK) and NF-κB cascade signaling pathways. Both in vivo studies in mice and in vitro studies in cultured human aortic endothelial cells (HAECs) and vascular smooth muscle cells (VSMCs) have shown that TMAO at physiological levels of TMAO induces the expression of cytokines and adhesion molecules. This activation is mediated, at least in part, by the NF-κB signaling pathway.50 TMAO directly stimulates hepatocytes to release hepatocyte-derived exosomes (Exos) that can be taken up by HAECs, thereby promoting the expression of inflammatory markers and apoptosis, a process that also proceeds through the NF-κB cascade signaling pathway.51 TMAO also indirectly augments oxygen radical production and activates sirtuin3-mitochondrial reactive oxygen species (SIRT3-mtROS), which recruits activated leukocytes to reach the vascular endothelium and induces cellular inflammatory factor production, further enhancing vascular inflammation.21 A study using liquid chromatography–tandem mass spectrometry, protein blotting, and fluorescent probes demonstrated that TMAO induces inflammation through this pathway in human umbilical vein endothelial cells (HUVECs) and aorta of ApoE−/− mice.52 This intestinal flora metabolite fosters the mitochondrial thioredoxin-interacting protein–NOD-like receptor protein (TXNIP-NLRP3) pathway and fuels the production of immune-inflammatory factors53;it can instigate the production of immune-inflammatory factors, such as IL-1, IL-18, and caspase-1, which disrupts the integrity of endothelial cells.54,55 There are cellular and mouse experiments confirming that TMAO stimulation induces the formation and activation of NLRP3 inflammasome complexes in carotid artery endothelial cells (CAECs). However, this formation and activation of inflammatory vesicles was eliminated in ECs pretreated with NLRP3 siRNA or the caspase-1 inhibitor WEHD, and TMAO led to the formation and activation of NLRP3 inflammasomes in ECs.54 In addition, TMAO triggers a decline in nitric oxide levels, leaving the endothelial function vulnerable to damage. In vitro studies have shown that NO expression is reduced in TMAO-treated endothelial progenitor cells (EPCs), suggesting that TMAO directly impairs NO-mediated endothelial function.56 TMAO causes vascular inflammation through these mechanisms, and its accumulation in vivo accelerates the progression of coronary atherosclerosis. Although these discoveries shed a light on the matter, further clarification of the mechanism of TMAO-engendered vascular inflammation could provide new targets for the treatment of coronary heart disease.
TMAO Induces Vascular Calcification
Vascular calcification is a pathological phenomenon prevalent in a variety of diseases. Calcification tends to cause a decrease in the elasticity of the wall of vessels, predisposing them to thrombosis, myocardial ischemia, heart failure, and stroke. A study conducted in vivo, ex vivo, and in vitro showed that TMAO upregulated the expression of bone-associated molecules including Runx2 (Runt-related transcription factor 2) and BMP2 (bone morphogenetic protein 2), indicating that TMAO promoted osteogenic differentiation of vascular smooth muscle cells and that TMAO positively regulated vascular calcification, an effect that was mediated by TMAO through activation of NLRP3 inflammasome and NF-κB signaling to promote vascular calcification.57 An in vitro study found that TMAO upregulated NF-κB signaling pathway, promoted lipogenic differentiation, inhibited osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs), and increased proinflammatory cytokine production and ROS release.58 Osteogenic endothelial progenitor cells (EPCs) contribute to endothelial repair of the damage and promote CAD and vascular calcification.59 One study on TMAO and vascular calcification in 179 patients with STEMI evaluated plasma TMAO levels using stable isotope dilution liquid chromatography–tandem mass spectrometry and the extent of vascular calcification through optical coherence tomography (OCT), finding a significant correlation between TMAO levels and the incidence of intimal calcification.60Because available investigations have only demonstrated a correlation between TMAO levels and endovascular calcification, the exact mechanism remains to be fully elucidated.
TMAO Stimulates the Conversion of Macrophages to Foam Cells
TMAO upregulates GRP97/HSP70 (glucose regulatory protein 97/heat shock protein 70), promotes lipid uptake by macrophages, and amplifies the expression of scavenger receptors, reducing cholesterol efflux from macrophages and increasing foam cell formation. Rodent studies demonstrated that TMAO increases the prevalence of atherosclerosis by increasing the expression of scavenger receptors and decreasing cholesterol efflux from macrophages, thereby increasing foam cell formation levels.14,61 A US animal study showed that animals supplemented with choline, TMAO, or betaine exhibited elevated levels of cluster of differentiation 36 (CD36) and scavenger receptor type A1 (SR-A1) macrophages and higher foam cell production compared with normal food.30 TMAO is similar to oxidized low-density lipoprotein (ox-LDL) and promotes the differentiation of monocytes into macrophages, the clearance of ox-LDL, and the conversion into foam cells.62 TMAO also intensifies the manifestation of vascular cell adhesion molecules (VCAMs) and stimulates macrophage adhesion through protein kinase C (PKC) and NF-κB signaling pathways.33 In short, TMAO accelerates atherosclerotic plaque formation by promoting macrophage adhesion and conversion to foam cells.
TMAO Enhances Platelet Reactivity and Promotes Thrombosis
Platelets are an essential component of the human blood system and are involved in coagulation and hemostasis processes. A disrupted vascular endothelium has slow blood flow and/or harbors generated vortexes, with the platelet hyperfunction susceptible to thrombosis, resulting in ischemia in vital organs. It has been demonstrated that TMAO increases platelet reactivity30,63,64 through a process potentially mediated through the promotion of ADP-induced Ca2+ release, which, in turn, is facilitated by the IP3 signaling pathway.65 A recent investigation revealed that an increase in TMAO results in promotion of tissue factor (TF) expression in the vascular endothelium, and thus, the TF-dependent prothrombotic effect is pronounced.66 A US study using dietary choline or TMAO fed to germ-free mice or animal models established by microbial transplantation confirmed the potential of TMAO production, a metabolite of the intestinal microbiota, in regulating platelet hyperresponsiveness and thrombosis.67 Animal studies have also shown that host liver FMO3 is the final step in the superorganismal TMAO pathway, involved in enhancing platelet reactivity and promoting thrombosis in vivo. TMAO also triggers the expression of vascular cell adhesion molecules, which can trigger thrombosis.68 Zhu et al69 found that choline increased fasting plasma TMAO levels and ADP-dependent platelet aggregation responses at 1 and 2 months of supplementation compared with baseline; however, aspirin treatment reduced the degree of TMAO elevation and platelet hyperreactivity. These discoveries provide a new basis for the treatment of cardiovascular disease and must remain to be further explored.
TMAO Disrupts Cholesterol Metabolism and Inhibits Bile Acid Synthesis
Reverse cholesterol transport (RCT), which has an antiatherosclerotic effect, mediates cholesterol efflux from foam cells and attenuates cholesterol deposition in the walls of vessels. Wang et al30 showed that TMAO inhibits RCT and disrupts cholesterol metabolism, accelerating plaque formation. The exact mechanism is unknown but could be linked to the promotion of cholesterol clearance receptor CD36 and SR-A1 expression.70 A US animal study showed that mice fed a TMAO-containing diet had a significant 35% reduction in reverse cholesterol transport compared with normal diet mice.25A report established no association between TMAO levels and cholesterol ester transfer protein (CETP) in patients with CAD.71 TMAO can also hinder bile acid synthesis. Under physiological conditions, hepatocytes synthesize primary bile acids, which are released in the intestine through the gallbladder and converted to secondary bile acids by the intestinal flora. Secondary bile acids are converted from primary bile acids catalyzed by intestinal bacterial enzymes and dehydroxylated by a debinding reaction and consist of deoxycholic acid and lithocholic acid. Secondary bile acids activate hormone-like signaling through the farnesoid X receptor (FXR) and pregnane X receptor (PXR), providing an effective anti-inflammatory effect. The activation of secondary bile acid receptors, such as FXR and PXR, is supposed to be associated with the slower development of coronary atherosclerosis in mouse models.72 In a study on ApoE−/− mice, TMAO altered the bile acid profile, accelerated the formation of aortic lesions and further FXR and small heterodimeric chaperones (SHP), decreased cholesterol 7α expression-hydroxylase (Cyp7a1), inhibited bile acid synthesis, and thus accelerated atherosclerosis formation.73 It has also been shown that perinatal administration of apoE-deficient mice with C-phycocyanin prevents atherosclerosis by regulating cholesterol and TMAO metabolism.74 Therefore, reducing circulating TMAO levels could diminish vascular inflammation and delay the formation of arterial plaques to some extent.
TMAO is a Possible Factor in Clopidogrel Resistance
Clopidogrel selectively inhibits the binding of ADP to the P2Y12 receptor on the platelet surface and the ADP-mediated activation of the glycoprotein GPIIb/IIIa complex, thereby constraining platelet aggregation. Clopidogrel has long played an irreplaceable role as an antiplatelet agent in the pharmacological treatment of ACS and postinterventional procedures. However, the medication has a high clinical resistance rate of up to 40%. In 1 animal study with 19 male rats, TMAO partially neutralized the effects of clopidogrel and induced resistance, and TMAO in circulation reduced the platelet aggregation inhibitory effect of clopidogrel by partially neutralizing its effect and inducing resistance.75 Possibly because TMAO and clopidogrel act on the same receptors, or their effects may be mediated in part by the same mechanisms, the specific mechanism through which TMAO leads to clopidogrel resistance is unclear and requires further in-depth examination.
Chromosomes 2q34 and 5q14 are Possibly Associated with Blood TMAO Levels
A 2-stage genome-wide association study (GWAS) identified 2 loci on chromosomes 2q34 and 5q14.1 that were significantly associated with plasma betaine levels in a cohort of patients undergoing selective cardiac evaluation. The 2q34 locus considerably affected the cascade of choline to urea cycle metabolism in women and was strongly linked to CAD risk in women, for example, in both male and female GenBank subjects, rs715 was primarily associated with reduced betaine and increased glycine levels, but the effect was stronger in women,76 which is consistent with the results of previous studies.77–79 The fact that TMAO is a metabolite of betaine and chromosome 2q34 and 5q14.1 loci could be indirectly associated with TMAO levels provides an opening for novel theories for the further reduction of cardiovascular disease risk, which should be explored in the future study.
REGULATING TMAO LEVELS TO MITIGATE CORONARY ARTERY DISEASE
High-fat diets, obesity, smoking, and diabetes are traditional risk factors for coronary heart disease, and choline-rich red meat, fish, shellfish, seafood, eggs, and dairy products induce or aggravate coronary heart disease by stimulating TMAO production; hence, lipid and choline intake must be reduced. The adverse effects of consuming eggs—which are rich in choline—and smoking seem to be cumulative, with egg intake alone generating about 60% of the effect of smoking alone.80 Half of dietary choline in Chinese adults comes from soy products.81Decreasing the ingestion of choline-rich foods, eating more fruits and vegetables, and exercising to lose weight benefits the body by decelerating the progression of CAD. One study found that combining a vegetarian diet with optimal drug therapy reduced body weight, total and LDL cholesterol levels, improved cardiometabolic risk factors, and altered the relative abundance of gut microbes and plasma metabolites in patients with ischemic heart disease.82 Some food components, such as 3,3-dimethyl-1-butanol (DMB), which can reduce plasma TMAO concentration; resveratrol, that inhibits TMAO production by booting the intestinal flora; and procatecholamines, a metabolite of anthocyanins, which inhibits coronary atherosclerosis, benefit the cardiovascular system.83
In contrast to all the above, some trials have demonstrated that suppressing choline intake does not necessarily result in cardiovascular benefits. One prospective inquiry reported that reducing betaine or choline intake did not result in the primary prevention of adverse cardiovascular events but that decreasing choline consumption possibly has adverse health effects.80Another investigation examining 95 patients with sepsis revealed that the impact of malnutrition reflected by plasma TMAO levels in these patients was far more significant than TMAO's proinflammatory effects.84 Taken together, reducing choline intake rather than eliminating it is beneficial to patients with cardiovascular disease to some extent.
Protecting the Intestinal Mucosal Barrier and Maintaining Intestinal Flora Balance
The intestinal mucosal barrier and intestinal flora balance are vital to the normal metabolic and immunomodulatory functioning of the body. In infancy, intestinal bacterial content is low; however, as the body matures and is constantly affected by the outside world, the number of intestinal microorganisms gradually increases, ultimately reaching a dynamic microbiota balance. If the intestinal mucosal barrier function is disrupted, the intestinal flora becomes susceptible to derivatives of various metabolic toxins, further causing cardiometabolic diseases.
The endogenous cannabinoid system (ECS) is an in vivo signaling system that belongs to the cardioprotective mechanism. The ECS can be stimulated and overactivated in the intestine by lipopolysaccharide (LPS), culminating in increased intestinal permeability and inflammatory response,80 impaired intestinal barrier function, and entry of intestinal flora-derived toxins into circulation, further damaging cardiac function. It has been reported that Akkermansia muciniphila improves coronary atherosclerosis by protecting the intestinal barrier function.85 Foods such as pumpkin, yogurt, honey, banana, and yam can enhance gastrointestinal motility, protect the gastric mucosal barrier, and maintain intestinal microenvironment balance. Long-chain monounsaturated fatty acids improve vascular endothelial function by altering the balance of microflora, while omega-3 fatty acids possibly prevent coronary atherosclerosis by alleviating endothelial function.86 TMAO production depends primarily on the intestinal flora; therefore, protecting the intestinal mucosal barrier and maintaining the dynamic balance of the intestinal flora could effectively delay the progression of coronary atherosclerosis.
Oral antibiotics can boost the cardiovascular system by improving intestinal microflora balance. Methimazole and indole inhibit FMO3, suppressing TMAO production and preventing coronary atherosclerosis.87Zhou et al88 found that applying antibiotics after myocardial infarction reduced systemic inflammation and myocardial injury. Wang et al observed no production of TMAO formation-promoting intestinal flora in mice treated with antibiotics.30In the examination by Wang et al., giving antibiotics to mice also did not yield TMAO formation-promoting intestinal flora, thereby mitigating TMAO's damage to cardiovascular function. Vancomycin has been reported to reduce myocardial infarct sizes in rats and also improve postinfarction myocardial function.89Clinical studies have demonstrated that broad-spectrum antibiotics such as metronidazole and ciprofloxacin can reduce TMAO levels.90In addition, combinedly administering vancomycin, neomycin sulfate, metronidazole, and ampicillin to mice also lessens TMAO levels,30 minimizing the risk of coronary atherosclerosis.
By contrast, the use of broad-spectrum antibiotics can equally significantly increase the risk of cardiac rupture and death after myocardial infarction,91 and the cumulative use of antibiotics by older women can amplify cardiovascular risk,92 possibly because antibiotics alter the composition of the intestinal flora, resulting in an inflammatory response.93Using antibiotics to treat cardiovascular disease remains contentious; therefore, their clinical application must be practiced cautiously to avoid disrupting intestinal flora homeostasis and lessen severe drug resistance issues.
Role of Prebiotics and Probiotics
Probiotics reduce TMAO levels and reverse the development of coronary atherosclerosis; for example, Lactobacillus decreases circulating TMAO levels and reduces atherosclerotic plaque formation in ApoE−/− mice.94
Clostridium perfringens improves the balance of intestinal flora and reduces TMA and TMAO levels, thereby preventing coronary atherosclerosis.95A study found Lactobacillus casei to diminish TMA production in mice expressing the human infant microbiota.96 Some probiotics exploit TMA and TMAO as substrates for methane production, decreasing TMA and TMAO levels in the body and the cardiovascular effects of TMAO.97,98 Probiotic bacteria are used as methane-generating substrates.
Some probiotics possibly harbor direct cardioprotective effects not linked to reducing the risk of adverse cardiovascular events through lowering TMAO concentrations.99Lactobacillus plantarum 299v supplementation improves vascular endothelial function and decreases inflammatory markers of stable CAD in men and is not associated with traditional risk factors and plasma TMAO concentrations through a mechanism probably related to its ability to increase L. plantarum 299v's ability to increase nitric oxide bioavailability and decrease systemic IL-8, IL-12, and leptin levels.100Probiotics containing Bifidobacterium bif195 have been shown to improve aspirin-caused gastrointestinal side effects.101Prebiotic and probiotic use benefit the composition of the human intestinal flora and may reduce the incidence of adverse cardiovascular events by lowering TMAO levels directly.
Fecal Matter Transplantation (FMT)
Fecal transplantation refers to the transplantation of feces from healthy individuals into the intestinal tract of patients with intestinal flora dysbiosis to re-establish intestinal flora balance. The mechanism through which this is perpetuated is one that possibly improves the intestinal environment of patients through the provision of beneficial bacteria from these fecal matters, enhances intestinal immune regulation, and ultimately achieves a dynamic balance in intestinal microorganisms. Tang et al found that fecal transplantation reduces TMA levels in feces, whereas Hu et al102 showed that FMT improves myocardial injury in mice with autoimmune myocarditis and rebalances the intestinal microbiota. Fecal bacterial transplantation could also be used to treat irritable bowel syndrome.103However, FMT does not only transplant beneficial fecal bacteria into a patient's intestine but also relocates harmful bacteria, making its use risky, which is why its clinical application is currently limited.
TMAO, as an intestinal flora derivative, is closely associated with cardiovascular disease, and the acknowledgment of this link is seeing an increase in research on intestinal flora and TMAO in cardiovascular disease. For coronary heart disease, reasonably adjusting diets, maintaining a dynamic intestinal flora balance, and probiotic and prebiotic supplementation can all prevent or delay the progression of the disorder (Fig. 2). Using antibiotics or FMT helps too but could cause adverse effects; their application in patients with CAD remains contentious. The mechanism of how TMAO causes coronary heart disease is still not fully understood and must be scrutinized further. More studies will be necessary to identify viable options for the treatment of coronary heart disease.
1. Dalen JE, Alpert JS, Goldberg RJ, et al. The epidemic of the 20(th) century: coronary heart disease. Am J Med. 2014;127:807–812.
2. van den Munckhof ICL, Kurilshikov A, Ter Horst R, et al. Role of gut microbiota in chronic low-grade inflammation as potential driver for atherosclerotic cardiovascular disease: a systematic review of human studies. Obes Rev. 2018;19:1719–1734.
3. Drosos I, Tavridou A, Kolios G. New aspects on the metabolic role of intestinal microbiota in the development of atherosclerosis. Metabolism. 2015;64:476–481.
4. Heianza Y, Ma W, Manson JE, et al. Gut microbiota metabolites and risk of major adverse cardiovascular disease events and death: a systematic review and meta-analysis of prospective studies. J Am Heart Assoc. 2017;6:1–28.
5. Senthong V, Wang Z, Li XS, et al. Intestinal microbiota-generated metabolite trimethylamine-N-oxide and 5-year mortality risk in stable coronary artery disease: the contributory role of intestinal microbiota in a COURAGE-like patient cohort. J Am Heart Assoc. 2016;5:1–8.
6. Heianza Y, Ma W, DiDonato JA, et al. Long-term changes in gut microbial metabolite trimethylamine N-oxide and coronary heart disease risk. J Am Coll Cardiol. 2020;75:763–772.
7. Dong ZX, Zhang J, Luo YC, et al. The correlation between trimethylamine N-oxide, lipoprotein ratios, and conventional lipid parameters in patients with unstable angina pectoris. Biosci Rep. 2020;40:BSR20192657-7.
8. Naqvi S, Asar TO, Kumar V, et al. A cross-talk between gut microbiome, salt and hypertension. Biomed Pharmacother. 2021;134:111156–111211.
9. Cheng TY, Li JX, Chen JY, et al. Gut microbiota: a potential target for traditional Chinese medicine intervention in coronary heart disease. Chin Med. 2021;16:108–120.
10. Gao J, Yan KT, Wang JX, et al. Gut microbial taxa as potential predictive biomarkers for acute coronary syndrome and post-STEMI cardiovascular events. Sci Rep. 2020;10:2639–2711.
11. Nishida A, Inoue R, Inatomi O, et al. Gut microbiota in the pathogenesis of inflammatory bowel disease. Clin J Gastroenterol. 2018;11:1–10.
12. Gulas E, Wysiadecki G, Strzelecki D, et al. Can microbiology affect psychiatry? A link between gut microbiota and psychiatric disorders. Psychiatr Pol. 2018;52:1023–1039.
13. Yamashita T, Yoshida N, Emoto T, et al. Two gut microbiota-derived toxins are closely associated with cardiovascular diseases: a review. Toxins (Basel). 2021;13:297–299.
14. Eshghjoo S, Jayaraman A, Sun Y, et al. Microbiota-mediated immune regulation in atherosclerosis. Molecules. 2021;26:179–211.
15. Yao ME, Liao PD, Zhao XJ, et al. Trimethylamine-N-oxide has prognostic value in coronary heart disease: a meta-analysis and dose-response analysis. BMC Cardiovasc Disord. 2020;20:7–9.
16. Zou D, Li Y, Sun G. Attenuation of circulating trimethylamine N-oxide prevents the progression of cardiac and renal dysfunction in a rat model of chronic cardiorenal syndrome. Front Pharmacol. 2021;12:1–12.
17. Emoto T, Hayashi T, Tabata T, et al. Metagenomic analysis of gut microbiota reveals its role in trimethylamine metabolism in heart failure. Int J Cardiol. 2021;338:138–142.
18. Taguchi K, Fukami K, Elias BC, et al. Dysbiosis-related advanced glycation endproducts and trimethylamine N-oxide in chronic kidney disease. Toxins (Basel). 2021;13:361–416.
19. Li Z, Hui J, Li S, et al. Trimethylamine N-oxide predicts stroke severity in diabetic patients with acute ischaemic stroke and is related to glycemic variability. Eur J Neurol. 2022:1–9.
20. Wang Z, Zhao Y. Gut microbiota derived metabolites in cardiovascular health and disease. Protein Cell. 2018;9:416–431.
21. Wang B, Qiu J, Lian J, et al. Gut metabolite trimethylamine-N-oxide in atherosclerosis: from mechanism to therapy. Front Cardiovasc Med. 2021;8:723886–723914.
22. Zhu Y, Jameson E, Crosatti M, et al. Carnitine metabolism to trimethylamine by an unusual Rieske-type oxygenase from human microbiota. Proc Natl Acad Sci U S A. 2014;111:4268–4273.
23. Leustean AM, Ciocoiu M, Sava A, et al. Implications of the intestinal microbiota in diagnosing the progression of diabetes and the presence of cardiovascular complications. J Diabetes Res. 2018, 20181220:1–9.
24. Ascher S, Reinhardt C. The gut microbiota: an emerging risk factor for cardiovascular and cerebrovascular disease. Eur J Immunol. 2018;48:564–575.
25. Koeth RA, Wang Z, Levison BS, et al. Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis. Nat Med. 2013;19:576–585.
26. Liu G, Cheng J, Zhang T, et al. Inhibition of microbiota-dependent trimethylamine N-oxide production ameliorates high salt diet-induced sympathetic excitation and hypertension in rats by attenuating central neuroinflammation and oxidative stress. Front Pharmacol. 2022;13:856914–857011.
27. Tang WW, Wang Z, Fan Y, et al. Prognostic value of elevated levels of intestinal microbe-generated metabolite trimethylamine-N-oxide in patients with heart failure: refining the gut hypothesis. J Am Coll Cardiol. 2014;64:1908–1914.
28. Cui X, Ye L, Li J, et al. Metagenomic and metabolomic analyses unveil dysbiosis of gut microbiota in chronic heart failure patients. Sci Rep. 2018;8:635–715.
29. Katsimichas T, Ohtani T, Motooka D, et al. Non-ischemic heart failure with reduced ejection fraction is associated with altered intestinal microbiota. Circ J. 2018;82:1640–1650.
30. Lee Y, Nemet I, Wang Z, et al. Longitudinal plasma measures of trimethylamine N-oxide and risk of atherosclerotic cardiovascular disease events in community-based older adults. J Am Heart Assoc. 2021;10:0206466-e20721.
31. Qi J, You T, Li J, et al. Circulating trimethylamine N-oxide and the risk of cardiovascular diseases: a systematic review and meta-analysis of 11 prospective cohort studies. J Cel Mol Med. 2018;22:185–194.
32. Schiattarella GG, Sannino A, Toscano E, et al. Gut microbe-generated metabolite trimethylamine-N-oxide as cardiovascular risk biomarker: a systematic review and dose-response meta-analysis. Eur Heart J. 2017;38:2948–2956.
33. Xie G, Yan A, Lin P, et al. Trimethylamine N-oxide-a marker for atherosclerotic vascular disease. Rev Cardiovasc Med. 2021;22:787–797.
34. Yang S, Li X, Yang F, et al. Gut microbiota-dependent marker TMAO in promoting cardiovascular disease: inflammation mechanism, clinical prognostic, and potential as a therapeutic target. Front Pharmacol. 2019;10:1360–1414.
35. Li J, Tan Y, Zhou P, et al. Association of trimethylamine N-oxide levels and calcification in culprit lesion segments in patients with ST-segment-elevation myocardial infarction evaluated by optical coherence tomography. Front Cardiovasc Med. 2021;8:628471–628478.
36. Mafune A, Iwamoto T, Tsutsumi Y, et al. Associations among serum trimethylamine-N-oxide (TMAO) levels, kidney function and infarcted coronary artery number in patients undergoing cardiovascular surgery: a cross-sectional study. Clin Exp Nephrol. 2016;20:731–739.
37. Senthong V, Wang Z, Fan Y, et al. Trimethylamine N-oxide and mortality risk in patients with peripheral artery disease. J Am Heart Assoc. 2016;5:e004237-e004238.
38. Taguchi K, Elias BC, Brooks CR, et al. Uremic toxin-targeting as a therapeutic strategy for preventing cardiorenal syndrome. Circ J. 2019;84:2–8.
39. Lee Y, Nemet I, Wang Z, et al. Longitudinal plasma measures of trimethylamine N-oxide and risk of atherosclerotic cardiovascular disease events in community-based older adults. J Am Heart Assoc. 2021;10:0206466-e20721.
40. Zhang P, Zou JZ, Chen J, et al. Association of trimethylamine N-oxide with cardiovascular and all-cause mortality in hemodialysis patients. Ren Fail. 2020;42:1004–1014.
41. Dong Z, Liang Z, Guo M, et al. The association between plasma levels of trimethylamine N-oxide and the risk of coronary heart disease in Chinese patients with or without type 2 diabetes mellitus. Dis Markers. 2018, 20188201:1–7.
42. Tang WHW, Wang Z, Li XS, et al. Increased trimethylamine N-oxide portends high mortality risk independent of glycemic control in patients with type 2 diabetes mellitus. Clin Chem. 2017;63:297–306.
43. Komaroff AL. The microbiome and risk for obesity and diabetes. JAMA. 2017;317:355–356.
44. Eyileten C, Jarosz-Popek J, Jakubik D, et al. Plasma trimethylamine-N-oxide is an independent predictor of long-term cardiovascular mortality in patients undergoing percutaneous coronary intervention for acute coronary syndrome. Front Cardiovasc Med. 2021;8:728724–728810.
45. Oakley CI, Vallejo JA, Wang D, et al. Trimethylamine-N-oxide acutely increases cardiac muscle contractility. Am J Physiology-Heart Circulatory Physiol. 2020;318:H1272–H1282.
46. Vernon ST, Tang O, Kim T, et al. Metabolic signatures in coronary artery disease: results from the bioHEART-CT study. Cells. 2021;10:980–1016.
47. Dai Y, Tian Q, Si J, et al. Circulating metabolites from the choline pathway and acute coronary syndromes in a Chinese case-control study. Nutr Metab (Lond). 2020;17:39–9.
48. Heyse M, Schneider C, Monostori P, et al. Trimethylamine-N-oxide levels are similar in asymptomatic vs. symptomatic cerebrovascular atherosclerosis. Front Neurol. 2021;12:617944–617946.
49. Meyer KA, Benton TZ, Bennett BJ, et al. Microbiota-dependent metabolite trimethylamine N-oxide and coronary artery calcium in the coronary artery risk development in young adults study (CARDIA). J Am Heart Assoc. 2016;5:0039700-e4011.
50. Seldin MM, Meng Y, Qi H, et al. Trimethylamine N-oxide promotes vascular inflammation through signaling of mitogen-activated protein kinase and nuclear factor-κB. J Am Heart Assoc. 2016;5:1–12.
51. Liu X, Shao Y, Tu J, et al. Trimethylamine-N-oxide-stimulated hepatocyte-derived exosomes promote inflammation and endothelial dysfunction through nuclear factor-kappa B signaling. Ann Transl Med. 2021;9:1670–1715.
52. Chen ML, Zhu XH, Ran L, et al. Trimethylamine-N-oxide induces vascular inflammation by activating the NLRP3 inflammasome through the SIRT3-SOD2-mtROS signaling pathway. J Am Heart Assoc. 2017;6:0063477-e6421.
53. Mills S, Stanton C, Lane JA, et al. Precision nutrition and the microbiome, part I: current state of the science. Nutrients. 2019;11:923–945.
54. Boini KM, Hussain T, Li PL, et al. Trimethylamine-N-oxide instigates NLRP3 inflammasome activation and endothelial dysfunction. Cell Physiol Biochem. 2017;44:152–162.
55. Sun X, Jiao X, Ma Y, et al. Trimethylamine N-oxide induces inflammation and endothelial dysfunction in human umbilical vein endothelial cells via activating ROS-TXNIP-NLRP3 inflammasome. Biochem Biophysical Res Commun. 2016;481:63–70.
56. Chou RH, Chen CY, Chen IC, et al. Trimethylamine N-oxide, circulating endothelial progenitor cells, and endothelial function in patients with stable angina. Sci Rep. 2019;9:4249–4310.
57. Zhang X, Li Y, Yang P, et al. Trimethylamine-N-oxide promotes vascular calcification through activation of NLRP3 (nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3) inflammasome and NF-κB (nuclear factor κB) signals. Arteriosclerosis, Thromb Vasc Biol. 2020;40:751–765.
58. Lin H, Liu T, Li X, et al. The role of gut microbiota metabolite trimethylamine N-oxide in functional impairment of bone marrow mesenchymal stem cells in osteoporosis disease. Ann Transl Med. 2020;8:1009–1010.
59. Toya T, Ozcan I, Corban MT, et al. Compositional change of gut microbiome and osteocalcin expressing endothelial progenitor cells in patients with coronary artery disease. PLoS One. 2021;16:02491877–e249216.
60. Barros PGMd, Li J, Tremblay C, et al. Cost modifications during the early years of the use of the national cardiovascular data registry for quality improvement. Clinics (Sao Paulo). 2020;75:e1708–e1717.
61. Collins HL, Drazul-Schrader D, Sulpizio AC, et al. L-Carnitine intake and high trimethylamine N-oxide plasma levels correlate with low aortic lesions in ApoE(-/-) transgenic mice expressing CETP. Atherosclerosis. 2016;244:29–37.
62. Holme SAN, Sigsgaard T, Holme JA, et al. Effects of particulate matter on atherosclerosis: a link via high-density lipoprotein (HDL) functionality?. Part Fibre Toxicol. 2020;17:36–12.
63. Martínez-Del Campo A, Romano KA, Rey FE, et al. The Plot Thickens: diet microbe interactions may modulate thrombosis risk. Cel Metab. 2016;23:573–575.
64. Wang Z, Roberts AB, Buffa JA, et al. Non-lethal inhibition of gut microbial trimethylamine production for the treatment of atherosclerosis. Cell. 2015;163:1585–1595.
65. Zhu W, Gregory JC, Org E, et al. Gut microbial metabolite TMAO enhances platelet hyperreactivity and thrombosis risk. Cell. 2016;165:111–124.
66. Witkowski M, Witkowski M, Friebel J, et al. Vascular endothelial tissue factor contributes to trimethylamine N-oxide-enhanced arterial thrombosis. Cardiovasc Res. 2021;118:2367–2384.
67. Vinchi F. Thrombosis prevention: let's drug the microbiome. Hemasphere. 2019;3:1655–1662.
68. Oikonomou E, Leopoulou M, Theofilis P, et al. A link between inflammation and thrombosis in atherosclerotic cardiovascular diseases: clinical and therapeutic implications. Atherosclerosis. 2020;309:16–26.
69. Zhu W, Wang Z, Tang WHW, et al. Gut microbe-generated trimethylamine N-oxide from dietary choline is prothrombotic in subjects. Circulation. 2017;135:1671–1673.
70. Stubbs JR, House JA, Ocque AJ, et al. Serum trimethylamine-N-oxide is elevated in CKD and correlates with coronary atherosclerosis burden. J Am Soc Nephrol. 2016;27:305–313.
71. Bordoni L, Samulak JJ, Sawicka AK, et al. Trimethylamine N-oxide and the reverse cholesterol transport in cardiovascular disease: a cross-sectional study. Sci Rep. 2020;10:18675–18679.
72. Zhang B, Wang X, Xia R, et al. Gut microbiota in coronary artery disease: a friend or foe?. Biosci Rep. 2020;40:1–11.
73. Ding L, Chang M, Guo Y, et al. Trimethylamine-N-oxide (TMAO)-induced atherosclerosis is associated with bile acid metabolism. Lipids Health Dis. 2018;17:286–288.
74. Coué M, Croyal M, Habib M, et al. Perinatal administration of C-phycocyanin protects against atherosclerosis in apoE-deficient mice by modulating cholesterol and trimethylamine-N-oxide metabolisms. Arteriosclerosis, Thromb Vasc Biol. 2021;41:e512-e523.
75. Ma R, Fu W, Zhang J, et al. TMAO: a potential mediator of clopidogrel resistance. Sci Rep. 2021;11:6580–6587.
76. Hartiala JA, Wilson Tang WH, Wang Z, et al. Genome-wide association study and targeted metabolomics identifies sex-specific association of CPS1 with coronary artery disease. Nat Commun. 2016;7:10558–10610.
77. Shin SY, Fauman EB, Petersen AK, et al., An atlas of genetic influences on human blood metabolites. Nat Genet. 2014;46:543–550.Spector TD, Soranzo N. An atlas of genetic influences on human blood metabolites.
78. Yu B, Zheng Y, Alexander D, et al. Genetic determinants influencing human serum metabolome among African Americans. Plos Genet. 2014;10:e1004212–e1004218.
79. Demirkan A, Henneman P, Verhoeven A, et al. Insight in genome-wide association of metabolite quantitative traits by exome sequence analyses. Plos Genet. 2015;11:e1004835–e1004839.
80. Meyer KA, Shea JW. Dietary choline and betaine and risk of CVD: a systematic review and meta-analysis of prospective studies. Nutrients. 2017;9:711–713.
81. Yu D, Shu XO, Rivera ES, et al. Urinary levels of trimethylamine-N-oxide and incident coronary heart disease: a prospective investigation among urban Chinese adults. J Am Heart Assoc. 2019;8:e010606-e010614.
82. Djekic D, Shi L, Brolin H, et al. Effects of a vegetarian diet on cardiometabolic risk factors, gut microbiota, and plasma metabolome in subjects with ischemic heart disease: a randomized, crossover study. J Am Heart Assoc. 2020;9:e016518-e016575.
83. Jonsson AL, Bäckhed F. Role of gut microbiota in atherosclerosis. Nat Rev Cardiol. 2017;14:79–87.
84. Chou RH, Wu PS, Wang SC, et al. Paradox of trimethylamine-N-oxide, the impact of malnutrition on microbiota-derived metabolites and septic patients. J Intensive Care. 2021;9:65–12.
85. Li J, Lin S, Vanhoutte PM, et al. Akkermansia Muciniphila protects against atherosclerosis by preventing metabolic endotoxemia-induced inflammation in apoe-/- mice. Circulation. 2016;133:2434–2446.
86. Tsutsumi R, Yamasaki Y, Takeo J, et al. Long-chain monounsaturated fatty acids improve endothelial function with altering microbial flora. Translational Res. 2021;237:16–30.
87. Xu J, Yang Y. Implications of gut microbiome on coronary artery disease. Cardiovasc Diagn Ther. 2020;10:869–880.
88. Zhou X, Li J, Guo J, et al. Gut-dependent microbial translocation induces inflammation and cardiovascular events after ST-elevation myocardial infarction. Microbiome. 2018;6:66–17.
89. Lam V, Su J, Koprowski S, et al. Intestinal microbiota determine severity of myocardial infarction in rats. FASEB J. 2012;26:1727–1735.
90. Tang WW, Wang Z, Levison BS, et al. Intestinal microbial metabolism of phosphatidylcholine and cardiovascular risk. N Engl J Med. 2013;368:1575–1584.
91. Tang TWH, Chen HC, Chen CY, et al. Loss of gut microbiota alters immune system composition and cripples postinfarction cardiac repair. Circulation. 2019;139:647–659.
92. Heianza Y, Zheng Y, Ma W, et al. Duration and life-stage of antibiotic use and risk of cardiovascular events in women. Eur Heart J. 2019;40:3838–3845.
93. Knoop KA, McDonald KG, Kulkarni DH, et al. Antibiotics promote inflammation through the translocation of native commensal colonic bacteria. Gut. 2016;65:1100–1109.
94. Qiu L, Tao X, Xiong H, et al. Lactobacillus plantarum ZDY04 exhibits a strain-specific property of lowering TMAO via the modulation of gut microbiota in mice. Food Funct. 2018;9:4299–4309.
95. Qiu L, Yang D, Tao X, et al. Enterobacter aerogenes ZDY01 attenuates choline-induced trimethylamine N-oxide levels by remodeling gut microbiota in mice. J Microbiol Biotechnol. 2017;27:1491–1499.
96. Martin FJ, Wang Y, Sprenger N, et al. Probiotic modulation of symbiotic gut microbial-host metabolic interactions in a humanized microbiome mouse model. Mol Syst Biol. 2008;4:157–215.
97. Velasquez MT, Ramezani A, Manal A, et al. Trimethylamine N-oxide: the good, the bad and the unknown. Toxins (Basel). 2016;8:326–411.
98. Goel JC, Gaur A, Singhal V, et al. The complex metabolism of trimethylamine in humans: endogenous and exogenous sources-CORRIGENDUM. Expert Rev Mol Med. 2016;18:199–203.
99. Moludi J, Alizadeh M, Lotfi Yagin N, et al. New insights on atherosclerosis: a cross-talk between endocannabinoid systems with gut microbiota. J Cardiovasc Thorac Res. 2018;10:129–137.
100. Malik M, Suboc TM, Tyagi S, et al. Lactobacillus plantarum 299v supplementation improves vascular endothelial function and reduces inflammatory biomarkers in men with stable coronary artery disease. Circ Res. 2018;123:1091–1102.
101. Mortensen B, Murphy C, O'Grady J, et al. Bifidobacteriumbreve bif195 protects against small-intestinal damage caused by acetylsalicylic acid in healthy volunteers. Gastroenterology. 2019;157:637–646.e4.
102. Hu XF, Zhang WY, Wen Q, et al. Fecal microbiota transplantation alleviates myocardial damage in myocarditis by restoring the microbiota composition. Pharmacol Res. 2019;139:412–421.
103. El-Salhy M, Mazzawi T. Fecal microbiota transplantation for managing irritable bowel syndrome. Expert Rev Gastroenterol Hepatol. 2018;12:439–445.